The use of braze joints on GE combustion end covers began in 1986. FIG. 1 is a cross-section of the first GE combustion end cover design with braze joints. This end cover 140, called MNQC, is constructed with five inserts 120 having 3 braze joints 100 each. Two of the braze joints are located at the same diameter, and the third is located at a different diameter. The braze joints are exposed to two different temperature ranges, 0 to 200.degree. F. and 675 to 725.degree. F., during operation of the gas turbine. This change in temperature in combination with an internal pressure of approximately 200 lb/in.sup.2 results in stresses at the braze joints 100. However, to date, this type of end cover has not experienced a failure at the braze joints under these stresses.
As combustion systems evolved over time, the end cover increased in complexity and now undergoes larger temperature changes and pressure forces. FIG. 2 represents a DLN combustion end cover 14 introduced in 1993. This type of end cover has as many as six inserts 12 having 4 braze joints each. As can be seen in FIG. 2, and as schematically shown in FIG. 3 (discussed below), the joints 10 are located at three diameters instead of two, which creates increased challenges for maintaining the machining tolerances. In addition, during operation of the gas turbine, a braze joint will be subjected to as many as four different temperature changes instead of two changes. The braze joints 10 are also exposed to a larger range of temperatures: 0 to 450.degree. F. and 675 to 800.degree. F. Moreover, the pressure force on the end cover has increased to 250 lb/in.sup.2. Thus, combustion end covers, in particular those made of 304L stainless steel and brazed with AMDRY 915, (Ni-13Cr-2.8B-4Si-4Fe) experience varying stress levels during field service depending upon their design. The increased complexity, and larger temperature changes and pressure forces of recent designs, have caused increased stresses at the braze joints.
By 1996, end cover failures began to occur with the end cover assemblies of the newer, more complex design. On break down, scorch marks were discovered on the failed end cover(s). Scorch marks on an end cover indicate a fuel leakage that could be the result of braze joint failure. To consider the impact of a braze joint on technical performance, one must consider the connection from the power plant to the gas turbine within it, to the combustion system of the gas turbine, to the end cover assembly. The end cover and inserts are connected with braze joints. If a braze joint fails, you get scorch marks. Braze joint failures could lead to catastrophic turbine failure, which could mean forced outages.
GE specification P9F-AG4, Acceptance Requirements, Brazing Vacuum Combustion Parts, originally issued Sep. 21, 1984 was used to specify the brazing process for combustion end covers. That specification did not, however, provide process specific requirements. GE specification P9F-AG7, Vacuum Furnace Brazing of Multi-nozzle Endcovers, was introduced May 24, 1996. P9F-AG7 introduced limited process parameters, in particular, a peak brazing temperature range of not less than 2075.degree. F. nor greater than 2150.degree. F., and a pre-braze diametrical gap size of 0.001-0.005 inches, common for either braze paste for braze foil processes.